1. Field of the Invention
The present invention relates to clean storage equipment for substrates and a method of storing substrates. In particular, the present invention relates to clean storage equipment for storing large substrates including semiconductor and glass substrates and a method of storing such substrates.
2. Description of the Prior Art
To make highly integrated fine devices from substrates or wafers, the surfaces thereof must not be contaminated with dust and atmospheric gas during manufacturing and storing stages.
In a cassette-to-cassette system, silicon wafers are stored in cassettes serving as carriers during a manufacturing stage. Each cassette holds 13 or 25 wafers and is set in and removed from a manufacturing apparatus such as a film forming apparatus and an etching apparatus. The cassettes with wafers used through the manufacturing stage are stored as they are in storage equipment.
The storage equipment may be a hermetic carrier box or a clean stocker.
The carrier box receives at least one cassette with wafers. The carrier box is kept in a clean room and is transported from a location to another as and when required. The carrier box is closed to prevent dust from contaminating the wafers.
The clean stocker generates dust-free airflow. FIG. 13A is a perspective view showing the inside of a clean stocker according to a prior art.
The stocker has a cassette transfer tool 505 that longitudinally travels. Many shelves 501 are arranged on each side of the cassette transfer tool 505.
A wafer cassette 502 holds wafers 503 and is inserted into an entrance 506. The cassette 502 is placed on the shelf 501 by the cassette transfer tool 505. To take the cassette 502 out of the stocker, the cassette transfer tool 505 moves the cassette 502 from the shelf 501 to an exit 507. The entrance 506 and exit 507 are each open, and the pressure of airflow from the inside of the stocker blocks outside air from entering the stocker.
FIG. 13B is a sectional view showing the stocker. Arrow marks indicate airflow in the stocker. The airflow is generated by fans 508 arranged under the shelves 501 and is passed through ducts 509, filters 510, shelves 501, and cassettes 502 toward the center of the stocker. From there, the airflow goes downwardly, is passed through filters 504, and is drawn by the fans 508, which again generates the airflow toward the ducts 509. In this way, the stocker circulates air to remove dust through the filters 510 and 504, thereby making clean air.
Generally, the wafer cassette 502 holds 13 or 25 wafers of 200 mm diameter, and the stocker stores 100 to 200 cassettes 502.
The carrier box mentioned first is easy to handle. The carrier box, however, usually has insufficient airtightness to allow outside air to enter as time passes. Then, wafers in the carrier box will be contaminated with water and impurity gas such as oxygen contained in the outside air.
On the other hand, the clean stocker may continuously drive the fans 508 to always circulate clean air through the stocker to prevent outside dust and impurity gas from entering the stocker. Even clean airflow, however, causes dust in the stocker if the airflow becomes turbulent. The turbulent airflow causes dust to dwell to make the filters 504 and 510 hardly remove the dust.
Accordingly, airflow around wafers in the stocker must be laminar. The movement of the cassette transfer tool 505, however, disturbs airflow in the stocker. To avoid this, the stocker must be large.
If the fans 508 stop due to power failure, outside air enters the stocker to deteriorate the cleanness of the stocker. To prevent this, the stocker must have an emergency power source. This increases the designing and manufacturing time and cost of the stocker.